This invention relates, in general, to semiconductor device assembly, and more particularly to methods and structures for connecting substrates.
Flip-chip technology is rapidly becoming an assembly technique of choice in applications such as smartcards. Flip-chip technology is used to connect a semiconductor chip to a thin printed circuit board to form a substrate assembly. This substrate assembly is then incorporated, for example, into a credit card device to provide a smartcard. Once incorporated into the credit card device, the semiconductor chip provides account tracking and security features for the cardholder.
In a typical smartcard flip-chip assembly process, a bumped semiconductor chip is bonded to a printed wiring or circuit board. The printed circuit board typically consists of a resin material with layers of patterned metal on both sides. The layers of patterned metal form contact pads and conductive traces. Metallized vias typically are used connect the metal layers.
A conductive adhesive is used to connect the bumps on the semiconductor chip to corresponding metal contact pads on the printed circuit board. The assembly is then heated to cure the conductive adhesive. Next, a non-conductive encapsulant or laminant is placed in gaps between the semiconductor die and the printed circuit board and between adjacent bumps. This is commonly referred to as an "underfill" process. The assembly is then heated again to cure the non-conductive encapsulant.
This process has several disadvantages. First, printed circuit boards having metal layers on both sides are expensive. Also, the process used to place the conductive adhesive in contact with the semiconductor bumps can have variations that result in inconsistent amounts of adhesive being applied. This can result in an inoperable device due to opens and/or shorts. In addition, the conductive adhesive curing process stresses the semiconductor die. This is due to, among other things, differences in the thermal expansion coefficients between the semiconductor die and the printed circuit board. Such stress can damage the semiconductor die. Additionally, the underfill process can be inconsistent thereby leaving voids, particularly at the conductive adhesive/non-conductive adhesive interface. Such voids can lead to reliability problems. Furthermore, the conventional smartcard flip-chip process is expensive because of its multiple steps.
As is readily apparent, methods and structures are needed for connecting substrates that are reliable, cost-effective, and efficient.